Electric toy vehicles have been a long-time favourite of young children ever since they are made available to the general consumers. Every year, numerous new types of toy come on to the toy market. Among the new comers, toys such as hand-held electronic video games have rapidly taken a large share of the market which was previously dominated by mechanical toys such as toy vehicles. In order to compete with the new generation of toys, it is necessary that the general performance and quality of modern toy vehicles must be elevated.
An important performance benchmark of a toy car is its ability to perform high speed cruising and manoevering. A toy car possessing high cruising speed and manuovering ability will almost certainly be preferred among young children for whom they are primarily designed. As a toy vehicle is typically a considerably scaled down version of a real vehicle, a cruising speed which is comparable to that of a road going vehicle would already appear to be exceedingly fast. Indeed, a toy vehicle having a cruising speed over 15 km/h is already classified as a racing toy vehicle bearing in mind that a conventional toy vehicle usually has a typical cruising speed of a few kilometers per hour.
Hitherto, most down-sized high speed vehicles are only available in the arena of model hobby vehicles which are primarily designated for adults and teenagers. In the USA and Europe, if a vehicle is legitimately to be offered to the general consumers as a `toy`, it must satisfy a series of stringent safety tests. One particular test which is widely agreed by toy manufacturers as the major barrier to the safety approval and therefore the development of high-speed toy vehicles is the test for compliance with ASTM HD271 (or EN 50088 for the European Union).
Under this test, the on-board motor of the vehicle is blocked, for example by wedging the driving wheels or by blocking the motor rotary shaft, while power is supplied to the driving motor. In the case of a remote-controllable vehicle, this is done by the remote controller which transmits a control signal to the receiver on-board the vehicle to supply power to the driving motor while the motor is blocked. Such a test, in every day life, simulates the accidental situation in which a careless child presses the rotating wheels of a toy vehicle against his body in order to feel the motion or to do other silly things.
In the absence of a fail-safe mechanism, an abnormally high electric current will flow through the motor as a result of the locked rotor if the rated power supply is still available to the motor. For a conventional low-speed vehicle having a small electric motor, this does not really matter since the fine motor core-windings would be burnt out almost instantly before the resulting heat can cause any appreciable damage.
For a high speed vehicle, however, the normal rated operating current is typically in the region of 4 to 5 A and the core winding current under blocked-rotor condition could surge to as high as 10 to 20 A. Such a high current, even with a relatively short duration, is already sufficient to burn the battery, melt the plastic bottom of a toy vehicle and may even cause fire. This is even more dangerous where the housing is metallic in which case heat conduction is much faster but less noticeable since the heat would not normally distort its appearance.
It is therefore desirable to provide a high speed toy vehicle having a fail-safe mechanism which would react promptly to a blocked-rotor situation and cut off power supply to the motor to prevent an accident as a result of over-current developed across an electric motor due to locking of rotary shaft under operating conditions. To be of practical industrial value, such a fail-safe mechanism must be relatively simple, reliable and in-expensive.
A simple conventional method to control motor over-current is, for example, by connecting the collector of a NPN-bipolar-transistor in series with the motor with the emitter connected to ground. When over current occurs, the transistor will be saturated and the drop in the collector-emitter voltage could be detected and used to cut off supply to the motor. This however would not be suitable for a more sophisticated toy vehicle having a controllable variable speed and with a high current rating motor.
Furthermore, the speed of a direct-current electric motor is usually varied by the amount of motor current. A modern approach for motor current control is usually not by means of power dissipating variable resistance but, instead, by controlling the pulse width of the supply current in order to obtain a variable average direct current level. As such, most conventional over-current control methods are not suitable.
According to the present invention, there is therefore provided a toy vehicle having an electric motor with a rotary shaft for driving said vehicle, comprising a motion tag, a motion sensor and a motion detection device, wherein said motion tag is movable in response to rotation of said rotary shaft, said motion sensor is adapted for non-contact detection of motion of said motion tag and generates a characteristic electrical signal output in response to movement of said motion tag, and said motion detection device accepts and processes said characteristic electrical signal output from said motion sensor and is adapted to trigger cut-off of power supply to said electric motor when said characteristic output signal indicates that there has been no relative motion between the motion tag and the motion sensor for a pre-determined duration.
Preferably the motion detection devices generates either a monotonously time-incremental or monotonous time decremental signal level output which is reset upon detection of said characteristic electrical signal output generated by said motion sensor in response to relative motion between said motion tag and said motion sensor, if said time-incremental or time-decremental signal output is not reset before reaching a predetermined threshold signal level output, the power supply to said electric motor will be cut-off.
Preferably said motion tag comprises a permanent magnet attached to the gear disc of the transmission system of said vehicle and said motion sensor comprises a plurality of coils.
Preferably said characteristic signal output from said motion sensor indicating motion comprises a pulsed signal.
Preferably said pulsed signal is converted into a signal notch during its passage through said motion detection device.